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Re: [Xen-devel] [PATCH 06/16] vmx: nest: handling VMX instruction exits

To: "Dong, Eddie" <eddie.dong@xxxxxxxxx>, Tim Deegan <Tim.Deegan@xxxxxxxxxxxxx>, "He, Qing" <qing.he@xxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH 06/16] vmx: nest: handling VMX instruction exits
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Wed, 15 Sep 2010 08:31:13 +0100
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On 15/09/2010 07:49, "Dong, Eddie" <eddie.dong@xxxxxxxxx> wrote:

>> What is wrong with simply extending x86_emulate to handle these
>> VMX-related instructions? We've dealt with emulators provided by
>> Intel guys in the past and frankly they were full of holes.
> Certainly fine to move those VMX instruction emulation to hvm/emulate.c as if
> you don't think that is VMX specific :)

It's the right place to put all instruction emulation, if at all possible.
You will then presumably require at least one or two call-back hooks to
caller context, at least to read/write VMCS, and that would be the place to
determine whether these VMX instructions are executable. For example, SVM
and PV emulation contexts would either leave the VMX callback hooks as NULL,
and/or there will be checks for is-nested-VMX-guest in the VMX callback
hooks, injecting #UD otherwise.

The main trick with x86_emulate extensions is determining the correct neat
small set of callback hooks to add, which is somewhat driven by deciding
what should be emulated within x86_emulate and what should be left without
for implementation in the caller's context.

 -- Keir

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